Field of the Invention
The present invention concerns a method for determining a control sequence for operating a magnetic resonance (MR) imaging system in order to generate magnetic resonance image data of a region to be imaged of an examination subject, wherein magnetic resonance raw data are acquired. The invention furthermore concerns a method for controlling a magnetic resonance imaging system in order to generate magnetic resonance image data of an examination subject, in which method magnetic resonance raw data are acquired. The invention also concerns a radio-frequency (RF) saturation pulse sequence determination system. The invention additionally concerns to a magnetic resonance imaging system.
Description of the Prior Art
In magnetic resonance imaging, it is often necessary to suppress unwanted signals in order to prevent the unwanted signals from being superimposed on the signals from a region that is to be imaged. In the imaging of the spine, for example, signals from moving internal organs and fat of the abdominal wall lead to severe motion and flow artifacts in the phase-encoding direction which obscure the part of the spine that is to be imaged. Fat signals of the abdomen/chest region have a similar effect on image quality in cardiac imaging. It is therefore necessary to saturate these regions which affect the imaging due to sources of interference during the imaging session in order to achieve good image quality and enable a meaningful diagnosis to be made. Typically, the region-specific saturation is achieved as follows. Prior to the actual pulse sequence for acquiring the raw data to be used for the magnetic resonance image generation, an RF pulse with a 90° flip angle is first generated that puts the signal of a specific region into the state of maximum transverse magnetization. Field gradients called spoiler gradients are then generated, by which the transverse magnetization of the region in question is nulled, thereby preventing it from influencing the subsequent imaging operation.
A successful design and application of an RF saturation pulse is dependent on a variety of factors:
One factor relates to anatomical accuracy. The more anatomically accurate the water or fat saturation proves to be, i.e. the more precisely a region from which spurious signals are to be expected is targeted by the water/fat saturation, the fewer spurious signals are to be expected, which leads to an improvement in image quality. What would be most effective, insofar as this factor is concerned, would be the application of RF saturation pulses by which arbitrarily shaped sub-areas of a region to be imaged FoV (Field of View) can be saturated.
A further factor relates to the B1+ sensitivity of the RF excitation coils of the magnetic resonance system. The RF excitation coils have a spatially varying excitation sensitivity that differs dependent on the object to be imaged and the region to be imaged. For this reason the spatial flip angle distribution of the saturation pulse can deviate significantly from the desired 90° angle. From this non-uniform distribution, a residual transverse magnetization results, which in turn causes unwanted artifacts, albeit in attenuated form.
Another factor relates to detuning effects known as off-resonance effects. Here, the static B0 field exhibits spatial deviations as a consequence of technical inaccuracies of the basic field magnet and patient-specific magnetic susceptibility. This results in unwanted frequency shifts, and phase errors develop that in turn adversely affect the accuracy and effectiveness of the RF saturation pulse.
Finally, the configuration of the RF saturation pulses is also influenced by the specific absorption rate (SAR). RF saturation pulses are associated with a high SAR, since a high flip angle of 90° is generated during the water/fat saturation. Furthermore, the RF saturation pulses must be set prior to each pulse sequence section, i.e. in each repetition time interval TR, which necessitates high repetition rates and leads to a very high SAR over a relatively long period of time.
Conventionally, broadband 1D RF saturation pulses are used for spurious signal suppression. The bandwidth of the saturation pulses is chosen so that fat and water signals will be nulled. A typical bandwidth value is in the region of 3.5 ppm. The saturation profile is one-dimensional, i.e. a slice-by-slice or slice-selective saturation is achieved. Consequently, many of these saturation pulses must be arranged manually in order to fit them to a desired anatomy. The conventional broadband 1D RF saturation pulses are associated with the following disadvantages. A lack of anatomical accuracy occurs even when the saturation pulses are aligned automatically. The broadband 1D RF saturation pulses are associated with a high SAR load, since very many successive pulses must be applied. The saturation is highly B1-sensitive, because the coil profile of the excitation coils is not taken into account as well during the design of the saturation pulses. There is often a failure to consider any type of information with respect to the anatomy, for example on the basis of existing auto-align algorithms (see, for example, U.S. Pat. No. 6,952,097 B2 or US 2003/0139659 A1). The advantages of the application of one-dimensional saturation pulses consist in the individual RF saturation pulses being of relatively short duration. Furthermore, the saturation is fairly robust against off-resonance effects owing to the wide bandwidth of the saturation pulses. Finally, the saturation of large-volume regions is extremely effective.
Alternatively, pulses called multidimensional spatially selective RF saturation pulses have been developed by which arbitrarily shaped regions can be saturated.
The application of such multidimensional spatially selective RF saturation pulses is described in Schneider et al, “Shaped Saturation with Inherent Radiofrequency-Power-Efficient Trajectory Design in Parallel Transmission”, in: Magnetic Resonance in Medicine, 2013, pp. 1-13, DOI:10.1002/mrm.25016.
The B1 excitation profiles and B0 inhomogeneities are also taken into consideration in the application of such saturation pulses, and arbitrary two-dimensional or multidimensional saturation patterns can be realized. However, these multidimensional spatially selective RF saturation pulses are also associated with disadvantages. The pulse length of the individual saturation pulses can be very long and consequently can increase the repetition time. This problem can be compensated by the introduction of a parallel transmission technology (pTX), as a result of which the pulses can be shortened. The pulses can only be shortened to a certain degree, however. RF wave-chain hardware limitations are the strongest limiting factor in this case. Significant reductions in RF pulse length are prevented as a result of this limitation, in particular when a fairly extensive region is to be saturated, as is the case, for example, in spine imaging.
Furthermore, the pulse bandwidth of the multidimensional spatially selective RF saturation pulses is by nature quite small. In spite of B0 inhomogeneities being taken into consideration in the RF pulse optimization process, the bandwidth, and consequently the robustness, of these pulses in respect of these effects is quite limited. In the imaging of the spine, for example, the B0 inhomogeneities can fluctuate at the respiratory frequency of 50 Hz. If the pulses are optimized with a wider bandwidth of the pulse frequency, the computing time and the RF pulse lengths are greatly increased.
An optimization of the pulses by means of a wider bandwidth is described in Setsompop et al., “Broadband Slab Selection with B+1 Mitigation at 7T via Parallel Spectral-Spatial Excitation”, in: Magnetic Resonance in Medicine, Issue 61, 2009, pages 492-500, DOI:10.1002/mrm.21834.
Furthermore, the design of the multidimensional spatially selective RF saturation pulses is dependent on the B1 profiles and the B0 inhomogeneity data, i.e. what is termed the adjustment data (Adj dat). Particularly in the imaging of the spine, no body matrix reception coil is used in order to suppress the signal from the abdominal wall. In this case it is not possible to acquire suitable adjustment data covering the entire abdominal region. This is due to the fact that, as a result of the low signal intensity of the signals from the abdominal wall region, particularly when very corpulent patients are examined, the signals from such a region are too weak. Consequently, the multidimensional spatially selective RF saturation pulses cannot be designed with sufficient accuracy to compensate for all the remaining spurious signals. However, the saturation with the aid of multidimensional, spatially selective RF saturation pulses also affords advantages. A high degree of anatomical accuracy is achieved in the saturation, wherein arbitrarily shaped saturation regions can be realized and the specification of the saturation regions can be combined well with existing auto-align algorithms. Accordingly, only a single pulse is required, rather than multiple pulses. The sequence is very SAR-efficient, i.e. the SAR load can be kept low. This can be achieved in particular in combination with the pTX technology. The values of the flip angles can be precisely predetermined over the entire region to be imaged. This can be achieved because the B1 profile is also incorporated into the pulse optimization process.